Police and the Department of Conservation were preparing to check nearby tramping huts at first light on Tuesday. Civil Defence advised there had been some self-evacuation by locals, but authorities have not ordered an evacuation.

A joint agency incident management centre, involving local and regional Civil Defence, police and DOC personnel, has been established at the Whakapapa Department of Conservation Visitor Centre in Tongariro National Park.

WITNESSES DESCRIBE EXPLOSIONS, ASH CLOUD

Police were alerted to the eruption shortly before midnight by a member of the public who reported seeing "flame-like explosions" and a cloud of ash coming from "a new hole in the side of the mountain".

They saw the eruption on Mt Tongariro's northern face, from his location on State Highway 46 near Lake Rotoaira, west of Rangipo.

Motorist Clayton Bolt told RadioLIVE a massive cloud was visible over the mountain, and was drifting toward the Napier region.

Truckie Tama Coker told Fairfax there was a "big flash", then it began "raining sand". Visibility was down to a matter of metres.

"I could just see the yellow glare on the mountain," Mr Coker told Fairfax. "I only had visibility of about 10 to 15 feet in front of me. It was a bit scary.

"It's something I'll probably never see again in my lifetime."

Richard Stevenson lives in the Napier suburb of Greenmeadows, about 200km away from Mount Tongariro:

"There's quite a strong sulphur smell," he told RadioLIVE. "We have washing out overnight because it was supposed to be a good day, so we had to redo that 'cause it smells, and we had a little bit of ash on the car."

Hamilton residents are also reporting a strong sulphuric aroma, with many using handkerchiefs to cover their faces outdoors.

John Ngawati lives on State Highway 46 in the shadow of Mt Tongariro, and made the decision to evacuate his family. He told RadioLIVE he was woken by a rumbling sound, and looked out to see the eruption in full flight.

"I went outside and there was this huge plume of ash," he says. "It looked like an electrical storm was going off in it. By this time the kids woke up, and it was time to get out of there."

Ann Lambert from the Rainbow Motel in Tokaanu says the eruption woke her overnight.

"I heard this very loud rumbling noise and I thought it was a truck, and I got up to see what on earth was going on," she says. "I pulled back the drape and saw this huge plume in the sky, with flames and sparks and things coming out of it."

Mrs Lambert says the plume was made up of smoke and dust and had "red flashes" that resembled flames. She initially thought Mt Ruapehu had erupted.

Truck driver Bryn Rodda told Radio NZ the ash on the Desert Road, State Highway 1, was thick with poor visibility before it was closed early on Tuesday.

"I could see this big cloud - it looked like a fist, basically, at an angle across the sky, and about the wrist section of the fist there was an orange ball of flash that I saw."

AVIATION ALERT RAISED, THEN LOWERED

GNS Science this morning raised its aviation colour code to red, meaning it is likely a significant amount of ash was emitted into the atmosphere, but has since pushed it back down to orange.

"Observations of Mt Tongariro this morning by GNS Science are that eruption activity has subsided," GNS reported just after 12pm.

"There have been no lahars or pyroclastic flows or lava flows."

The eruption itself is believed to have lasted only about a minute or two.

Prime Minister John Key told Firstline this morning the eruption was "fairly moderate, but there's been ash and rock expelled from Tongariro."

If the situation deteriorated, he said Civil Defence would take control.

"Civil Defence take responsibility of that and they run the full operation, so at some point if it progressed to that level then the normal activation comes in.

"Basically they take over and have responsibility for everyone in assuring that all the emergency services and the like are deployed."

FLIGHTS DISRUPTED

Several Air New Zealand flights to airports east of Mt Tongariro, including Gisborne, Rotorua, Taupo and Napier, along with Palmerston North, were delayed or cancelled following the eruption, affecting flights to and from Auckland, Wellington and Christchurch.

The airline says it will not fly any planes through ash.

"We will not fly through ash and are constantly taking guidance from the Civil Aviation Authority (CAA) and the MetService to ensure we can continue to carry passengers where safe routes and altitudes are available," says Capt David Morgan, general manager of airline operations and safety.

Hawke's Bay Airport has reportedly closed until further notice.

Police initially closed the Desert Road (State Highway 1) between Rangipo and Waiouru and State Highway 46 west of Rangipo, both of which have now reopened. Police received reports of light ash falling on both roads.

They had also received reports of ash falling as far east as State Highway 5 near Te Haroto and onto Napier itself.

Kevin Cannell, chair of National Park School's board of trustees says classes are running today, but there will be no skiing because of rain.

Police say motorists should expect delays.

Napier Civil Defence is urging residents to sign up to its text alert service by sending 'follow napier_cd' to 8987. The alerts only work on the Vodafone and Telecom networks, however.

BUSINESS AS USUAL FOR SKI FIELDS

Manager of the Turoa ski field on Mt Ruapehu Chris Thrupp says they're open for business.

"At this stage [we] are open to business as usual this morning," says Mr Thrupp. "We’ve had no advice from the Department of Conservation, police or GNS Science that there is any increased risk on Mt Ruapheu. They are three independent volcanoes and we are some distance away.

"Ash is probably the concern at this stage if the wind direction changes, which we don’t think it will, and blows ash towards Ruapehu - then the problem we’ll have to deal with is ash on the snow."

He says people visiting the area are anxious, but for the locals, its "reasonably normal… Eruptions aren't uncommon."

Whakapapa ski field is also unaffected by ash at this stage.

Huts and crossings on the mountain have been closed by DOC. Whanganui area manager Nick Peat said they would be working with police and local iwi Ngati Tuwharetoa and taking a "precautionary" approach.

LINK TO OTHER VOLCANIC ACTIVITY?

GNS volcanologist Michael Rosenberg has ruled out any link between last night's eruption and recent activity at another volcano, White Island in the Bay of Plenty.

Mr Rosenberg is concerned there was no warning a full eruption was coming.

"It's unexpected, but it seems to have been a fairly small eruption," he told RadioLIVE. "We're not particularly worried at the moment because the eruption seems to have quietened down - obviously it can change at any time.

"All we can do is monitor the volcano by monitoring the seismic activity."

Because Mt Tongariro has been quiet for so long, no one really knows what could happen next.

"This has taken us by surprise," says Mr Rosenberg. "It's gone from some little earthquakes that seemed to be tailing off, then all of a sudden this has gone boom.

"We really didn't expect that there would be an eruption apparently out of nowhere… It's possible that this could be the start of a much longer episode, or alternatively it could be the end. We just don't know."

Volcanologist Brad Scott says the eruption was driven by steam, not molten rock. There were changes noted in the composition of gases coming out of Mt Tongariro in the week leading up to the eruption, but no increase in background seismic activity.

"We're not seeing more energy being released, and we didn't see a lot of energy being released before the eruption," says Mr Scott, who wouldn't rule out further events.

"I wouldn't be surprised if there are more small-scale hydrothermal-driven eruptions."

The weather is preventing flights over the mountain to see exactly what has happened, but it's suspected the Te Maari craters are involved.

TONGARIRO FACTS AND FIGURES

The Tongariro volcano is a massive complex of volcanic cones formed by at least 12 vents erupting over more than 275,000 years

Mt Tongariro itself is a complex of craters that have been active at different periods

Ngauruhoe, the central 2291-metre peak, is the youngest and highest cone in the complex and the most active, last erupting in 1977

Ngauruhoe is regarded as the main vent of the complex

Mt Tongariro is the northern-most of the three volcanoes, with Mt Ruapehu the southern-most.

Until Monday night, the last time Mt Tongariro erupted was in 1896-97, when the upper Te Maari crater erupted, dumping 50mm of ash on the Desert Road, and wafting ash as far as Napier

In 1868, violent earthquakes marked the eruption that formed the upper Te Maari crater, named after a Maori chieftainess.

Geologists describe Tongariro as an "active stratovolcano", or composite cone, made up of alternating layers of ash and lava flow.

GNS Science has four seismographs, one microphone, two GPS stations and chemical and gas monitors on Tongariro

Two web cameras face Mt Ngauruhoe to monitor volcanic activity

Tongariro in Maori means "fire carried away or seized by the cold south wind"

Tongariro National Park was given to the nation in 1887 by chief Te Heuheu Tukino IV

It is the country's oldest national park and is one of the country's most popular destinations for tramping and day walks

Parts of the Lord of the Rings trilogy were filmed in the park, where Mt Ngauruhoe was cast as Mt Doom.